To mount accessory parts, e.g., bipods, lamps, sights, cameras and the like, quickly and repetitively accurately on handheld firearms, so-called Picatinny rails are often used. However, these generally standardized mounting rails or other suitable adapters are, as a rule, integrated into the weapon housing or stationarily attached on a front stock or on other parts of the weapon. As a result, these mounting devices are not very flexible and, in addition, cannot be easily mounted and dismounted.